In recent years, metal halide lamps for irradiating light of ultraviolet rays are utilized in a wide variety of fields such as a printing process, a painting process and a resin sealing process. As metal halide lamps for use in these processes, there have hitherto been developed lamps capable of producing light of higher illumination level in order to efficiently carry out the treatments such as printing, painting and sealing in a short period of time. A high-pressure mercury lamp is a main current of a light source but there has been known a metal halide lamp of which luminous efficiency in the ultraviolet region is higher than that of the high-pressure mercury lamp. A metal halide lamp includes an arc tube into which metals are sealed as halides to produce light of a spectrum peculiar to metals.
The inventors of the present application are aware of the following patent literatures concerning such metal halide lamps for use in irradiating light of ultraviolet rays. Relevant parts in the respective patent literatures will be cited and mentioned.